The manufacture of acrylic polymers such as cast poly(methyl methacrylate) has increased substantially due to growing demand. With increasing use of acrylic polymers, much effort has been made to find polymerization systems which will improve production rates without compromising the quality of the finished product.
Acrylic polymers are generally prepared by casting a blend of methyl methacrylate monomer containing some polymer, commonly referred to as prepolymer or prepoly syrup. A variety of initiators and initiator systems have been suggested and used to increase the production rate of the resulting cast poly(methyl methacrylate) sheet. Many have been combinations of a variety of peroxide and azo initiators. Dual initiator systems of different peroxide/peroxide, peroxide/azo, and azo/azo initiators have been suggested, including those found in Novak, U.S. Pat. No. 4,328,329 issued May 4, 1982. Novak was concerned with finding a combination of initiators which would provide fast polymerization with low residual monomer. To do so, he selected a dual initiator system consisting of an active peroxide and less active azo. The active peroxide (t-butyl or t-pentyl peroxyneodecanoate) insured fast polymerization. The less active azo(2,2'-azobis(isobutyronitrile)) reduced the residual monomer to acceptable levels. Together, these initiators provided a combination which seemed to work well.
Other initiator systems previously used include that described by D. W. Wood in Plastics Engineering, May 1975, pp. 51-53. Wood discloses the combined use of t-butyl peroxyneodecanoate and decanoyl peroxide. However, Novak found that a peroxide/azo initiator system was superior to the dual peroxide system.
It has been found that acrylic sheet produced in accordance with the Novak patent, while generally of excellent quality, was prone to bubble formation when heated. This problem was particularly severe when the acrylic sheet was thermoformed at temperatures at about or above 200.degree. C. since gas solubility decreases with increasing temperature. The bubble formation was aggravated by nitrogen gas which is emitted as the azo initiator decomposes. The nitrogen is less soluble in acrylic sheet than such gases as carbon dioxide and will cause bubbles to form at lower concentrations. This problem is less likely to occur with peroxides which emit carbon dioxide when they decompose. The carbon dioxide is more soluble than nitrogen and is less likely to cause bubbling either during polymerization or subsequently during thermoforming of the acrylic sheet. However, the decomposition properties of the peroxides need to be selected carefully since too much carbon dioxide can also cause bubbling and adversely affect thermal stability.
Continuing effort has been directed to providing an initiator system which will result in a fast production rate and produce sheet free of cavities or other defects, relatively free of residual monomer and which has high thermal stability so that it can be subsequently thermoformed at temperatures at or above 200.degree. C. without bubble formation.